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Published byAlice Gordon Modified over 9 years ago
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STEMS
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Purpose of Stems Support leaves Transport water and nutrients Store water and food
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WHY?!? Growth in height only occurs at tip of roots and branches
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Specialized Stems Cactus –is a stem that stores food and water for the plant –spines are leaves of a cactus
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Rhizomes –underground stem which grows horizontally through soil –Ex. Iris Specialized Stems
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Bulbs –specialized stems that store food –Ex. onions, tulips, daffodils Specialized Stems
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Corms –Short, swollen underground plant stem used as food storage –Ex. crocuses, gladiolas Stolons –slender stem that grows above ground –“touch, root-and- shoot” –Ex. strawberries Specialized Stems
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Where do Stems Grow? Apical Meristems –cell division occurs at tip of stem Lateral Meristems –cell division occurs –stems grow in diameter
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Structure of stems Nodes –region on stem where 1 or more new leaves form Internode –distance between segments Lenticels –pores in surface of stem –allow gas exchange
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Buds –Terminal - bud at the end of stem –Lateral - buds on side of stem –Bud scales- protective covering over embryonic shoots (present in winter) Structure of stems Lateral Bud
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Primary Growth in Stems Epidermis –outer layer –protection –prevents H 2 0 loss –contains lenticels allow for O 2 and CO 2 exchange Cortex –lies inside epidermis –storage of food for stem
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Primary Growth in Stems Pith –located in center of stem –stores food Vascular Bundles –Xylem : transfers H 2 O –Phloem : transfers food
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Type of stems Monocot Stem –**V.B. are scattered** –xylem in center –phloem on outside
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Types of Stems Dicot Stem –**V.B. make a circle** –xylem closer to center –phloem behind xylem
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Secondary Growth in Stems Occurs mainly in dicots b/c monocots lack lateral meristems Stems increase in diameter due to lateral meristems –2 types : vascular cambium, cork cambium
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Vascular cambium –makes new xylem, phloem through cell division –this becomes secondary xylem & secondary phloem Secondary Growth in Stems
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Wood –is secondary xylem Heartwood (pith) –is older primary xylem –stopped transporting H 2 0 Secondary Growth in Stems
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Sapwood –new secondary xylem –lighter in color (still transports H 2 0) Bark –protection for woody stems –made up of cork, cork cambium, phloem
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Why does bark appear to be rough or crack? Cells aren’t living –cork cambium produces cork, but dies before maturity –as tree grows, cork ruptures and forms cracks
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Annual rings Springwood –xylem produced in spring –rain is plentiful –cells are larger than summer wood Summerwood –xylem still produced –limited amt. of water –cells are smaller than springwood
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Annual rings –during hibernation xylem isn’t produced –difference from 1 year to the next –can tell app. age of tree –tell environmental conditions –lg.= moist – sm. = drought
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